Rebels loyal to South Sudan’s former vice president claim they’ve captured an oil refinery in Upper Nile state that is near one of the country’s largest working oil fields.

In a statement, rebel spokesman James Dak said they’d seized the refinery near Paloch oil field to deprive the government of oil revenues used to finance the country’s ongoing civil war.

The rebels, who support Riek Machar, had vowed last month to capture key oil installations in an effort to dislodge President Salva Kiir.

Ariane Quentier, spokeswoman for the United Nations’ Mission in South Sudan, said she was aware of the rebels’ claim but could not confirm it.

The government depends entirely on revenue from the few oil fields still operating, said Jok Madut Jok, an analyst at the Juba-based Sudd Institute. "If the rebels take control of the actual productive oil fields, then they could probably bring the war to a close very quickly. There is no other source of income for the government."

None of the refineries is working, but the rebels’ alleged action could frighten and erode the confidence of foreign oil companies operating there, Jok said.

"It will have repercussions," Jok said. "… Because the closer the rebels get to the production fields, the more likely [it is] that the oil companies will have to halt production and take their people out of harm’s way."

Dak said the rebels had asked foreign oil companies – including China National Petroleum Corp., India's ONGC Videsh and Malaysia's Petronas – to remove their staff and close the fields to avoid damage.

Despite several cease-fire agreements, South Sudan has experienced a surge in violence in recent weeks. Rebel forces launched an assault Friday in Malakal, an oil town and the Upper Nile’s state capital. The East African bloc IGAD, or Intergovernmental Authority on Development, said government troops have been targeting civilians and committing human rights abuses.

Last week, the United Nations and international aid groups evacuated their humanitarian staff from Unity State, next to Upper Nile, leaving hundreds of thousands in need of food and medical services.

The conflict, which began in December 2013 when Machar tried to unseat Kiir, has displaced roughly 1.8 million people, the U.N. has reported. Thousands have been killed, though no reliable figure is available.